1. Field of the Invention
The present invention relates generally to injection devices for use with an external infusion system whereby a desired fluid is subcutaneously delivered to a patient, and more particularly to a disposable injection set for delivering insulin to the patient which injection set is conveniently and inexpensively manufactured of materials which will not cause a reaction with insulin passing therethrough, thereby avoiding the adverse results of such an insulin reaction.
Generally, in order to subcutaneously dispense a fluid from an external source to a patient, the distal end of a hollow needle is inserted through the skin of the patient, thereby providing a passageway to the desired subcutaneous injection location under the skin of the patient. The proximal end of the hollow needle located externally of the skin of the patient is connected to one end of a tube, the other end of which is connected to the external source of the fluid to be injected, typically with a luer connector made of hard PVC (polyvinyl chloride) and which may be easily connected by merely inserting the mating connectors together and twisting to lock them together.
A preferred technique for connecting the tube to the needle involves the molding of a hard PVC segment around the proximal end of the needle, and then utilizing flexible PVC tubing which may be solvent bonded to the hard PVC segment molded around the proximal end of the needle. Solvent bonding is preferred because of the relative ease of the solvent bonding operation, the strength and durability of the solvent bond, and the inexpensive cost of solvent bonding. Since such infusion sets are disposable, cost and acceptable shelf life are important criteria by which such an infusion set will be judged.
The recent popularity of insulin infusion pumps as an alternative to multiple daily injections for insulin-dependent diabetics requires the use of such an injection set to deliver insulin from a small, portable insulin infusion pump to the subcutaneous injection location. It has been determined that there exists a substantial problem with the use of injection sets as described above in that flexible PVC is not completely insulin compatible. This is in contrast to hard PVC, which is perfectly safe for use with insulin. While the exact nature of the reaction exhibited by insulin in contact with flexible PVC has not been determined with certainty, it is believed that the insulin, which is pH sensitive, reacts with CO.sub.2, the flow of which therethrough is not inhibited by flexible PVC. In addition, the large quantities of plasticizer used in flexible PVC may result in a leaching problem when used with insulin.
Since flexible PVC is not a barrier for CO.sub.2, the CO.sub.2 which flows through the flexible PVC tubing will react with the insulin, causing the insulin to aggregate and to precipitate out of solution. Such precipitation of the insulin will likely cause clotting and blockage in the tube or in the needle, thereby inhibiting the flow of insulin to the subcutaneous depot.
Heat will also accelerate the clotting process of insulin in flexible PVC tubing without the pH change caused by CO.sub.2. The reason for this has not been finally determined, but it may be due to zinc in the insulin forming zinc chloride. In any event, heat will further compound the situation faced by delivery of insulin through flexible PVC tubing.
The amount of insulin exiting the injection set will therefore vary considerably, with portions of the insulin becoming attached to the interior of the tube and eventually coating the interior of the tube even if blockage does not occur. Over time, the situation will improve somewhat assuming blockage of the tube per se does not occur, but the amount of insulin actually delivered to the patient will vary considerable even with the best of circumstances. It may therefore be appreciated that the use of a flexible PVC tubing injection set to deliver insulin from an insulin infusion pump is neither desirable nor medically acceptable.
It should also be noted that other substances exhibit reactions when delivered through flexible PVC tubing. Lipids and proteins have adverse reactions with flexible PVC delivery systems, and nitroglycerin also reacts to some degree with a flexible PVC environment.
One solution which has been discovered to the problem has been through the use of polyethylene tubing, which does not cause a reaction with insulin passing therethrough. Polyethylene is a barrier to CO.sub.2, and the major problem of CO.sub.2 passing through the tubing is thereby eliminated. Additionally, the problem of clotting of the insulin due to heat is also substantially minimized.
However, polyethylene is not solvent bondable as is flexible PVC, and a substantial problem in manufacturing of injection sets using polyethylene has arisen. The preferred method to date has been to use epoxy to "pot" the polyethylene tubing to the needle, without using a hard PVC segment molded onto the needle. Although the hard PVC segment could be used, since polyethylene is not susceptible to solvent bonding it would be necessary to epoxy the polyethylene tubing to the hard PVC segment, resulting in a higher cost injection set.
Several problems have arisen with the use of epoxy bonded polyethylene infusion sets, all of which are due to the relative disadvantage of the epoxy bonding process to the solvent bonding process. First of all, an epoxy bond is simply not as strong as a solvent bond. Secondly, epoxy bonds have substantial aging problems, which limit shelf life of the injection set. Since the epoxy bond loses its mechanical bonding properties over time, the injection set will become less sturdy, with the potential for the tubing coming loose from the needle increasing substantially over time. Thirdly, batch control of epoxy used in epoxy bonding is time consuming and cumbersome. Finally, epoxy bonding or "potting" is a more expensive process than solvent bonding, resulting in a product having an economic disadvantage relative to a product made by solvent bonding.
It is thereby apparent that there exists a substantial need for an injection set for delivery of insulin (or other fluids exhibiting reactions when flowed through flexible PVC tubing), which injection set utilizes polyethylene tubing to inhibit reaction and subsequent degradation of insulin flowing therethrough. It is a primary objective that the injection set be susceptible to manufacture by solvent bonding, thereby resulting in a superior mechanical bond having great strength and excellent shelf life.
It is also desirable that the improved injection set be of economical manufacture, to thereby result in an inexpensive disposable product which may be easily marketed. Finally, it is desirable that the improved injection set achieve the aforementioned advantages and solve the previously mentioned disadvantages without substantial disadvantage.